Robert Lunn wrote:
>
> >I need to measure a continuous frequency between 0 - 250 cycles/MINUTE,
> >and transmit it via RS-232 at 2400 baud in a message containing 6 bytes
> >once 5 seconds.
> >
> >The PIC I plan to use is the low power 84 variant, running at 32 KHz,
> >if possible.
>
>         Peter, you don't indicate the accuracy with which you need
>         to measure the frequency...
>
>         However, with a 32.768 kHz clock (which is 8192 instructions
>         per second) a variation of one instruction in detecting the
>         edge of a 250 rpm signal will cause an error in the calculated
>         instantaneous frequency of 0.2 rpm.

+/- 0.5 rpm is fine.

>
>         Assuming you don't want your measurement to be much less
>         accurate than this, you must measure the edges of the input
>         signal to within an instruction time.
>
>         You can do this by setting up TMR0 as a free running timer
>         with no prescale, configuring RB0 as an interrupt, and
>         connecting your input signal to RB0.
>
>         On each rising edge you get an interrupt, and in your ISR
>         you capture TMR0.
>
>         You must handle overflow of TMR0 in your main line code.
>
>         You are now measuring your signal period, and thus your
>         signal frequency, as accurately as it is possible.

Seems a good solution so far.


>
>         Ok, now to the output side of the equation...
>
>         I can solve your problem if I can change your requirements :-)
>
>         Use 300 baud instead of 2400 baud (which is barely possible
>         anyway).

300 baud may be a bit slow, Ill have to check. 1200baud is preferred.
I want to use a 32KHz crystal because of its availability. Maybe I'll
have to use a faster one, so as to increase the baud rate. The reason
for the slow crystal is purely for power consumption reasons.


>
>         At 300 baud you have just over 27 instructions per bit.  The
>         ISR to capture TMR0 should be six instructions.  This interrupt
>         can never occur more than once per character (indeed, can never
>         occur more than once per six byte transmission).  Stretching
>         a single bit time by 20% will _probably_ be ok...
>
>         Don't forget to handle TMR0 overflow between each byte.
>
> ___Bob


Thanks again,

Peter.
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Peter Homann   email: peterh@adacel.com.au       Work : +61 3 9596-2991
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